Compounds such as cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate have been used as cellulose ester supports in photographic light-sensitive materials. Cellulose triacetate has been popularly used due to its excellent characteristics such as dimensional stability, transparency, luster.
A process for producing cellulose triacetate film according to a solution process is conducted as follows. Cellulose acetate usually having a bound acetic acid content of 56% or more is dissolved in a mixed solvent of a lower hydrocarbon chloride (for example, methylene chloride) and a lower aliphatic alcohol (for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, or the like). A proper plasticizer and, in some cases, a dye are added thereto. The resulting solution is subjected to purifying steps such as filtration, defoaming, and adjustment of viscosity (concentration), temperature, etc., then fed to a filming machine. The purified solution is extruded in a film form onto a moving support by means of a suitable film-forming machine (this procedure being generally referred to as casting procedure, and the moving support being a surface of rotary drum or endless belt). The solvent is evaporated from the solution film during almost one rotation of the support. The thus formed film is delaminated from the support surface and dried in a suitable drying machine to evaporate the remaining solvent from the delaminated film.
Therefore, when producing cellulose triacetate film, it is quite advantageous from the industrial point of view that the cellulose triacetate starting material readily form a uniform solution, that the cellulose triacetate solution permit a plasticizer and a dye added thereto to be readily and uniformly mixed therein, that the solution can be easily filtered and defoamed, that the solution can be easily extruded from the film-forming machine onto the moving support and that the solvent used can be easily evaporated from the solution film.
One approach for attaining the above-described advantages is the reduction of the viscosity of the cellulose triacetate solution.
In general, the viscosity of the cellulose triacetate solution can be reduced by decreasing degree of polymerization of the cellulose triacetate starting material and/or by decreasing the concentration of its solution.
However, reduction of the degree of polymerization of the cellulose triacetate starting material is accompanied by deterioration of the mechanical strength of the film prepared therefrom. Therefore, the above-described process is unfavorable.
However, a decrease in the concentration of the cellulose triacetate solution requires a high drying temperature and/or a long drying time in the solvent-evaporating step to be conducted after the casting procedure. Due to the increased use of energy and time this is disadvantageous method of production.
Thus, a cellulose triacetate solution with a high viscosity is usually used.
Therefore, currently conducted production steps involve the following defects.
(1) Stirring efficiency, when preparing the cellulose triacetate solution, is so poor that it takes a long time to obtain a uniform solution.
(2) Upon addition of a plasticizer and a dye to the solution, a long time is required for attaining uniform mixing.
(3) Piping must be completed using pipes with a large diameter due to great resistance of the solution in the transporting pipes. Therefore, a wide space is required for equipment, leading to serious equipment disadvantage. In addition, a solution-ejecting pump with a large amount of power must be used for transporting the solution, leading to disadvantage in production cost.
(4) In filtering the solution to remove foreign matter (non-soluble cellulose triacetate, dust, etc.), an increased pressure must be applied. Thus, a torch, pressure-resistant equipment and a large-power, solution-ejecting pump must be used. This is disadvantageous from the point of view of equipment and production cost. With respect to safety, it should be noted that the application of an increased filtering pressure can result in gushing out of the solution.
(5) When defoaming the solution, the temperature of the solution is raised and/or a pressure is applied thereto, which leads to the same disadvantage as described in (4). Defoaming the solution by allowing it to stand requires a long period of time.
(6) When extruding the solution from a film-forming machine onto a moving support, an increase in extrusion speed causes disturbance in stream of said solution in the film-forming machine, and the surface of solution film is difficult to be leveled. Thus the resulting film can not be practically used. The resulting film is particularly unsuited as a support for photographic light-sensitive material. Therefore, the extrusion speed must be reduced to such a degree that no disturbance takes place in the solution, which is industrially quite disadvantageous. The above-described disadvantages are also encountered when using other cellulose esters.